ath5k: fix some comment typos

Fix comment about dma sizes, brackets were missing. Replace 'insure' with
'ensure'.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/ath/ath5k/ath5k.h b/drivers/net/wireless/ath/ath5k/ath5k.h
index cf16318..387c120 100644
--- a/drivers/net/wireless/ath/ath5k/ath5k.h
+++ b/drivers/net/wireless/ath/ath5k/ath5k.h
@@ -566,7 +566,7 @@
 )
 
 /*
- * DMA size definitions (2^n+2)
+ * DMA size definitions (2^(n+2))
  */
 enum ath5k_dmasize {
 	AR5K_DMASIZE_4B	= 0,